git: kernel - Fix bug in machdep.pmap_mmu_optimize
dillon at crater.dragonflybsd.org
Tue Oct 31 10:49:58 PDT 2017
Author: Matthew Dillon <dillon at apollo.backplane.com>
Date: Fri Oct 20 23:43:15 2017 -0700
kernel - Fix bug in machdep.pmap_mmu_optimize
* Fix a bug in the pmap_mmu_optimize feature (default disabled). When
enable this feature will automatically share page table pages with
equivalent permissions for objects covering the whole page table page.
* However, the code which cleaned out the 'old' page table page failed
to properly lock its pindex across the replacement operation, likely
allowing threaded programs to sometimes race it and potentially
lose track of one or more PTEs.
The code tried to hold onto proc_pd_pv to prevent races, but there is
still a small window due to the fact that pmap_allocpte() allocates
pv's from the leaf upward. So if the pmap optimization is good in
one thread but the fails in another for the same page table (for
example, due to a vm_map_entry split), a race can ensue.
* Use the existing pt_placemarker feature to properly lock the empty
page table page slot while it is being replaced.
* Add a soft-locking mechanism to temporarily work around a
(usually short-lived) allocation live-lock which can crop up
when one thread is trying to replace a page table page while
another is trying to allocate to it.
Summary of changes:
sys/platform/pc64/include/pmap.h | 1 +
sys/platform/pc64/x86_64/pmap.c | 86 +++++++++++++++++++++++++++++++++-------
2 files changed, 73 insertions(+), 14 deletions(-)
DragonFly BSD source repository
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